In the heating of solid bodies to incandescence, this non-visual emission is the necessary basis of the visual.  A platinum wire is stretched in front of the table, and through it an electric current flows.  It is warmed by the current, and may be felt to be warm by the hand.  It emits waves of heat, but no light.  Augmenting the strength of the current, the wire becomes hotter; it finally glows with a sober red light.  At this point Dr. Draper many years ago began an interesting investigation.  He employed a voltaic current to heat his platinum, and he studied, by means of a prism, the successive introduction of the colours of the spectrum.  His first colour, as here, was red; then came orange, then yellow, then green, and lastly all the shades of blue.  As the temperature of the platinum was gradually augmented, the atoms were caused to vibrate more rapidly; shorter waves were thus introduced, until finally waves were obtained corresponding to the entire spectrum.  As each successive colour was introduced, the colours preceding it became more vivid.  Now the vividness or intensity of light, like that of sound, depends not upon the length of the wave, but on the amplitude of the vibration.  Hence, as the less refrangible colours grew more intense when the more refrangible ones were introduced, we are forced to conclude that side by side with the introduction of the shorter waves we had an augmentation of the amplitude of the longer ones.

These remarks apply not only to the visible emission examined by Dr. Draper, but to the invisible emission which precedes the appearance of any light.  In the emission from the white-hot platinum wire now before you, the lightless waves exist with which we started, only their intensity has been increased a thousand-fold by the augmentation of temperature necessary to the production of this white light.  Both effects are bound up together:  in an incandescent solid, or in a molten solid, you cannot have the shorter waves without this intensification of the longer ones.  A sun is possible only on these conditions; hence Sir William Herschel’s discovery of the invisible ultra-red solar emission.

The invisible heat, emitted both by dark bodies and by luminous ones, flies through space with the velosity of light, and is called radiant heat.  Now, radiant heat may be made a subtle and powerful explorer of molecular condition, and, of late years, it has given a new significance to the act of chemical combination.  Take, for example, the air we breathe.  It is a mixture of oxygen and nitrogen; and it behaves towards radiant heat like a vacuum, being incompetent to absorb it in any sensible degree.  But permit the same two gases to unite chemically; then, without any augmentation of the quantity of matter, without altering the gaseous condition, without interfering in any way with the transparency of the gas, the act of chemical union is accompanied by an enormous diminution of its diathermancy, or perviousness to radiant heat.